How to Use Waker and Context in Rust Futures

You use Context to register a Waker that notifies the executor when a future is ready to resume, and you implement poll_next to return Poll::Pending if data isn't ready yet. The Context passed to your poll method contains the Waker you must call via waker.wake() when the asynchronous operation completes.

use std::pin::Pin;
use std::task::{Context, Poll, Waker};

struct MyStream;

impl Stream for MyStream {
    type Item = String;

    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        // If data is ready, return it
        if let Some(item) = self.get_ready_data() {
            return Poll::Ready(Some(item));
        }

        // Otherwise, register the waker and return Pending
        cx.waker().wake_by_ref(); // Register interest
        Poll::Pending
    }
}

Think of a Future as a task that isn't finished yet. The Context is the manager that tells the task when to pause and how to be notified later. The Waker is the phone number the task gives the manager so the manager can call it back when the work is done.